30Sand Casting - ProsProduce parts with complex geometries, both internally and externally.Possible to net shape with no further manufacturing required.Large parts can be produced.Wide choice of metals.Suitable for mass production.

31Sand Casting - Cons PorosityPoor dimensional control for some processesPoor surface finish for some processesLimitation on mechanical propertiesSafety hazardEnvironmental hazard

35Casting Quality - DefectsMicroporosity -Network of small voids caused by localised solidification shrinkage. Caused by the freezing manner of the alloy.Hot tearing - Occurs at location with high stress due to inability to shrink naturally. Resolve by mold collapsing or removing from the mold immediately after freezing.

37Casting Quality - Defects (Sand Casting)Penetration - penetration of molten metal into the sand. Harder packing of sand is needed.Mold shift - shift of the cope relative to the drag.Core shift - shift of the core, usually vertical.Mold crack - mold strength insufficient, liquid metal form a fin of the final casting.

46Investment CastingFIGURE Schematic illustration of investment casting (lost-wax process). Castings by this method can be made with very fine detail and from a variety of metals. Source: Steel Founders’ Society of America.

51Expendable Pattern CastingFigure Schematic illustration of the expendable pattern casting process, also known as lost foam or evaporative casting. Pattern made from Polystrene and vaporized when in contact with molten metal. The pattern can include the sprue and runner. No cope / drag is needed.

53Expendable Mold Casting - Expanded Polystrene MoldPros:Pattern need not be removed.No cope /drag is needed, all features are built into the pattern.Possibility for automated production.Cons:The pattern is not reusable.

54Operation Sequence of Making a Ceramic MoldFIGURE Sequence of operations in making a ceramic mold.

56Expendable Mold Casting - Plaster Mold and Ceramic MoldSimilar to sand casting in terms of process.Plaster mold is for lower temperature alloys while ceramic mold is for higher temperature alloys.Pros:Good surface finish and dimensional control.Capability to make thin cross sections.Cons:Curing takes too long to render it unsuitable for volume production.

57ٍShell Casting Dump-Box TechniqueFigure A common method of making shell molds. Called dump-box technique, the limitations are the formation of voids in the shell and peelback (when sections of the shell fall off as the pattern is raised). Source: ASM International.

60Expendable Mold Casting - Vacuum Mold1. Thin pre-heated plastic sheet sucks onto the vacuum vented pattern surface.2. Special flask with vents and filled with sand is placed over the pattern.3. Another plastic sheet covers the sand and vacuum is drawn.

61Expendable Mold Casting - Vacuum Mold4. Vacuum on the pattern is released to free the sand mold.5. Cope and drag is assembled to form the complete mold. The plastic sheet is burnt away when in contact with the molten metal.

62Vacuum-Casting ProcessFIGURE Schematic illustration of the vacuum-casting process. Note that the mold has a bottom gate. (a) Before and (b) after immersion of the mod into the molten metal. Source: After R. Blackburn.

63Expendable Mold Casting - Vacuum MoldSand held together by vacuum pressure.Pros:Sand can be recovered unlike shell mold.No chemical binder, and therefore no special treatment for the sand.No water mixed with the sand and therefore no moisture related problemsCons:Relative slowNot readily adaptable to mechanization.

68Permanent Mold Casting - Die CastingHot-chamber machines :Metal molten in container attached to machine. Typical injection pressures are 7 to 35 MPa. The piston is subjected to the melting temperature of the metal and thus the process is often used for low melting point metals such as zinc, tin, lead or magnesium alloys.Cold-chamber machinesMolten metal is poured into an unheated chamber from an external container. Typical injection pressures are 14 to 140 MPa. Often used for high melting point metal such as aluminum, brass, and magnesium alloys.

69Die Casting in Hot-Chamber ProcessFIGURE Sequence of steps in die casting of a part in the hot-chamber process. Source: Courtesy of Foundry Management and Technology.

70Die Casting in Cold-Chamber ProcessFIGURE Sequence of operations in die casting of a part in the cold-chamber process.

73Permanent Mold Casting - Die CastingMold made of tool steel.Mold opening mechanism to be synchronized with ejector pins.Venting is needed for air and gas typically at the parting surface.Flash formation is common.

75Centrifugal Casting ProcessFIGURE Schematic illustration of the centrifugal casting process. Pipes, cylinder liners, and similarly shaped parts can be cast by this process.

76Semicentrifugal Casting ProcessFIGURE (a) Schematic illustration of the semicentrifugal casting process. (b) Schematic illustration of casting by centrifuging. The molds are placed at the periphery of the machine, and the molten metal is forced into the molds by centrifugal forces.